Ultrasound images of various parts of the body, and in particular cardiac ultrasound images, may contain pools of blood in, for example, arteries or cardiac chambers, which blood pools are surrounded by tissue. Such images are frequently degraded by the presence of "clutter" which can be defined as extraneous image echoes displayed in blood pool areas. The presence of such clutter effectively lowers the contrast between blood and tissue, making it harder to find true physical tissue boundaries.
There are many causes of clutter, some of which can be addressed by the design of the system. However, there are also many patient-dependent factors involved in clutter, resulting in very different clutter levels from one patient to the next, these patient-dependent factors being poorly understood, and, therefore, difficult to eliminate.
At present, to the extent the clutter problem is dealt with at all in ultrasound imaging systems, an attempt is made to process the raw or received image in order to reduce the apparent clutter. This processing usually takes the form of a non-linear intensity mapping function which suppresses or rejects low level echoes, the assumption being that echoes resulting from clutter will be at a lower level than echoes resulting from tissue. However, this is not necessarily a valid assumption. Although a properly set threshold level for such non-linear intensity mapping will reduce or eliminate clutter, it will also degrade the presentation of tissue, since the speckle pattern making up tissue contains many echoes at or below the intensity of the clutter, even though the average intensity of the tissue is generally higher. Therefore, such reject processing is not particularly effective in increasing the contrast between blood and tissue.
A need, therefore, exists for an improved method and apparatus for suppressing clutter in an ultrasound image of a body portion containing both blood pool areas and tissue areas, without any significant degradation in the tissue portion of such image.